The present invention relates to a lenticular lens sheet used in the screen of for example rear projection type television sets.
Conventionally, as the screen of a rear projection type television set, a combination of a lenticular lens sheet and a Fresnel lens sheet is known. In such screens, two or more lens sheets made of plastics of different thickness and material are combined. The magnitude of elongation and contraction of the lens sheets in response to ambient temperature/humidity changes is different, and this may therefore result in the occurrence of a gap between the lens sheets. Accordingly, as shown in FIG. 5, occurrence of a gap between the lens sheets is prevented by causing the two lens sheets to adhere by utilizing the resilience of the lens sheets, by applying warping to the lenticular lens sheet and Fresnel lens sheet such that the degree of warping is more gentle in the case of the Fresnel lens sheet 2 than in the case of the lenticular lens sheet 1. Applying the warping to the lens sheet is commonly effected by placing a flat lens sheet in a prescribed mold and then heating it. The degree of warping (arcuate curvature R) of the lenticular lens sheet is the same both in the vicinity of the center and in the vicinity of the upper and lower edges. In the mode of use of these two lens sheets as a screen, as shown in FIG. 4, which is given in explanation of the present invention, the upper edges and lower edges are held fixed by a screen frame 6. When such lens sheets to which warping has been applied are assembled in a television set using a screen in which they are fixed in a frame, and this is transported by being loaded onto for example a truck, rubbing may occur between the two lens sheets due to vibration etc during transportation. Rubbing was particularly likely to occur at the upper and lower edges of the screen, where scraping off of the top portions of the Fresnel lens or the lenticular lens could severely impair image quality.
As counter-measures for this, there have been proposed a screen (see Japanese Patent Application Laid-open No. H2-93531), in which the top portions of the Fresnel lens are flattened, and a screen (see Japanese Utility Model Application Laid-open No. H1-97327) shaped approximately as a spherical surface in which the cross-sectional shape of the lenticular lens sheet (warped shape) is a combination of curved lines and straight lines.
However, when making the top portions of the Fresnel lens flat as a countermeasure for rubbing between the lenticular lens sheet and Fresnel lens sheet, in order not to impair the image quality of the screen, the restriction had to be imposed of flattening only the minute portion at the Fresnel lens tips through which the emitted light did not pass, without altering the shapes of the other portions, through which the emitted light passed. Typically, therefore the width of the flat part which was provided was only a few xcexcm, which was insufficient to offer the benefit of preventing scraping due to rubbing of the two lens sheets.
In a circular Fresnel lens sheet, the lens grooves are arranged concentrically. Also, the metal mold for a circular Fresnel lens sheet is manufactured by machining for example a brass plate using a cutting tool; typically, machining is performed with respect to one groove while rotating the brass plate or the like. Consequently, it is not possible to provide the flat parts of the Fresnel lens top portions selectively exclusively in the vicinity of the upper and lower edges of the screen, and even regions in the vicinity of the center line (height direction) of the lens sheet offset towards the left and right edges, where flat parts of the Fresnel lens top portions are basically unnecessary, must be machined; there was thus the problem that considerable time was required for machining of the metal mold.
On the other hand, in the case of a screen shaped approximately as a spherical surface in which the cross-sectional shape of the lenticular lens sheet (warped shape) was a combination of curved lines and straight lines, in a high-humidity environment, it was found that a gap occurred between the lenticular lens sheet and Fresnel lens sheet in the region where the cross-sectional shape of the lenticular lens sheet was linear. In addition, in regions where the cross-sectional shape (warped shape) of the lenticular lens sheet was curved, scraping sometimes occurred of the Fresnel lens top portions and lenticular lens top portions due to rubbing between the two lens sheets during transportation etc. It was therefore not possible to solve simultaneously the problems of eliminating a gap between the lenticular lens sheet and Fresnel lens sheet and of preventing scraping due to rubbing of the two lens sheets.
The present invention was made in view of the above problems. An object thereof is to provide a lenticular lens sheet wherein, even when a screen is constructed by combining this with a Fresnel lens sheet, there is little occurrence of gaps between it and the Fresnel lens sheet and there is little occurrence of scraping of the lens top portions due to rubbing against the Fresnel lens sheet. In a lenticular lens sheet (1A) according to the present invention that solves the above problem, wherein
the cross-sectional shape of the lenticular lens sheet in the vertical direction of the screen has warping, such that the optical output lens side is the concave side, and such that its upper part and lower part are of roughly symmetrical arcuate shape with respect to the center line (CL in FIG. 1) in the height direction of the lenticular lens sheet,
considering a chord (B1) that connects two points (P2, P3) to each other which are points on the approximate curve of arcuate shape of this lenticular lens sheet and are of distance 150 mm from this center line (CL) and between which a distance is 300 mm, a distance to this chord (B1) from the point (P1) where this approximate curve and this center line intersect is represented by S0,
considering a chord (B2) that connects an upper edge (UE) of the approximate curve of this lenticular lens sheet to a point (P4) on the approximate curve of the lenticular lens sheet at a distance of 300 mm from the upper edge (UE) in the downwards direction, a distance to this chord (B2) from the point (P6) where the perpendicular from the central point (P5) of this chord (B2) intersects the approximate curve of this lenticular lens sheet is represented by S1,
considering a chord (B3) that connects a lower edge (LE) of the approximate curve of this lenticular lens sheet to a point (P7) on the approximate curve of the lenticular lens sheet at a distance of 300 mm from the lower edge (LE) in the upwards direction, the distance to this chord (B3) from the point (P9) where the perpendicular from the central point (P8) of this chord (B3) intersects the approximate curve of this lenticular lens sheet is represented by S2, respectively, and
S0, S1 and S2 satisfy the following expression (1) and expression (2):
S0 greater than S1 greater than 0xe2x80x83xe2x80x83(1)
S0 greater than S2 greater than 0xe2x80x83xe2x80x83(2).
Herein, the approximate curve of the arcuate shape of the lenticular lens sheet is expressed as the curve of the arcuate shape obtained when the lenticular lens sheet is gradually lowered onto a suitable board from a condition in which it was suspended with the left side face (or the right side face) of the lenticular lens sheet supported so that no force is applied thereto in the horizontal direction (i.e. a condition rotated by 90xc2x0 from its condition of use), and the side face of the lenticular lens sheet on the opposite side to the side face that is supported abuts this board.
Preferably, in addition to the above expressions (1) and (2), the magnitude of the warping expressed by the above S0, S1 and S2 satisfies the following expressions (3) to (5):
5(mm)xe2x89xa6S0xe2x89xa625(mm)xe2x80x83xe2x80x83(3)
0(mm) less than S1 less than 25(mm)xe2x80x83xe2x80x83(4)
0(mm) less than S2 less than 25(mm)xe2x80x83xe2x80x83(5).
Also, a screen according to the present invention comprises a lenticular lens sheet as described above and a Fresnel lens sheet arranged on the optical input side face of this lenticular lens sheet, this Fresnel lens sheet having warping such that its optical output side is concave and in which this warping is more gentle than the warping of this lenticular lens sheet.
FIG. 2 shows a diagrammatic perspective view of a lenticular lens sheet according to the present invention and a Fresnel lens sheet combined with this. As shown in FIG. 2, typically lenticular lens sheet 1A according to the present invention is provided with warping such that the optical output side where the optical output side lens 3 and black stripes 4 are provided constitutes the concave side and the optical input side lens 5 constitutes the convex side. FIG. 3 shows a diagram of the respective warping shapes of a lenticular lens sheet according to the present invention and a Fresnel lens sheet combined with this.
As shown in FIG. 3, although the Fresnel lens sheet 2 that is combined with lenticular lens sheet 1A has warping such that its optical output side is a concave side, the degree of this warping is more gentle than that of lenticular lens sheet 1A. Specifically, if, for the optical input side face of the lenticular lens sheet 1A, the distance from the tangent L1 in the vertical direction of the screen at its center to the upper and lower edges of the lenticular lens sheet 1A is taken as D1 and, for the optical input side face of the Fresnel lens sheet 2, the distance from the tangent L2 in the vertical direction of the screen at its center to the upper and lower edges of the Fresnel lens sheet 2 is taken as D2, and the magnitude of the warping is expressed by D1 or D2, D2 s preferably {fraction (1/20)} to ⅓ of D1.
For the lenticular lens sheet according to the present invention, the warping (S1, S2) nearer to the upper and lower edges is smaller than the warping (S0) in the middle region of the lenticular lens sheet.
When a lenticular lens sheet and Fresnel lens sheet are combined as a screen, as shown in FIG. 4, their upper edges and lower edges are fixed to and supported by a screen frame 6. With the lenticular lens sheet of the present invention, in which the warping (S1, S2) nearer the upper and lower edges is smaller than the warping (S0) in the middle region of the lenticular lens sheet, the pressing force acting at the points of contact between the lenticular lens sheet surface and the Fresnel lens sheet surface in the vicinity of the screen frame at the upper and lower edges can be lowered, making it possible to suppress scraping of the Fresnel lens top portions occurring in this region. Also, in the vicinity of the center of the lens sheet, where a gap may easily occur between the lenticular lens sheet and Fresnel lens sheet, the pressing force between the two lens sheets can be made high, so occurrence of gaps between the lenticular lens sheets and Fresnel lens sheets is infrequent, just as in the case where flat lenticular lens sheets are employed to which warping is not applied.